The provocative question in the title of this post is prompted by this notable op-ed by Jacob Sullum at Forbes headlined "Economists Predict Marijuana Legalization Will Produce 'Public-Health Benefits'." Here are excerpts:

In their 2012 book Marijuana Legalization: What Everyone Needs to Know, Jonathan Caulkins and three other drug policy scholars identify the impact of repealing pot prohibition on alcohol consumption as the most important thing no one knows. Are cannabis and alcohol complements, so that drinking can be expected to increase along with pot smoking? Or are they substitutes, implying that more pot smoking will mean less drinking? For analysts attempting to calculate the costs and benefits of legalizing marijuana, the question matters a lot, because alcohol is considerably more dangerous than marijuana by most measures. If the two products are complements, states that legalize marijuana can expect to see more consumption of both, exacerbating existing health and safety problems. But if the two products are substitutes, legalizing marijuana can alleviate those problems by reducing alcohol consumption.

Reviewing the evidence in the Journal of Policy Analysis and Management, Montana State University economist D. Mark Anderson and University of Colorado economist Daniel Rees find that “studies based on clearly defined natural experiments generally support the hypothesis that marijuana and alcohol are substitutes.” [Study Here] Increasing the drinking age seems to result in more marijuana consumption, for instance, and pot smoking drops off sharply at age 21, “suggesting that young adults treat alcohol and marijuana as substitutes.” Another study found that legalizing marijuana for medical use is associated with a drop in beer sales and a decrease in heavy drinking. These results, Anderson and Rees say, “suggest that, as marijuana becomes more available, young adults in Colorado and Washington will respond by drinking less, not more.”

That conclusion is consistent with earlier research in which Anderson and Rees found that enacting medical marijuana laws is associated with a 13 percent drop in traffic fatalities. [Study Here] That effect could be due to the fact that marijuana impairs driving ability much less dramatically than alcohol does, although the fact that alcohol is more likely to be consumed outside the home (resulting in more driving under its influence) may play a role as well....

Anderson and Rees note that UCLA drug policy expert Mark Kleiman, who co-wrote Marijuana Legalization and has been advising Washington’s cannabis regulators, recently described a worst-case scenario for legalization featuring an increase in heavy drinking, “carnage on our highways,” and a “massive” increase in marijuana consumption among teenagers. “Kleiman’s worst-case scenario is possible, but not likely,” they conclude. “Based on existing empirical evidence, we expect that the legalization of recreational marijuana in Colorado and Washington will lead to increased marijuana consumption coupled with decreased alcohol consumption. As a consequence, these states will experience a reduction in the social harms resulting from alcohol use. While it is more than likely that marijuana produced by state-sanctioned growers will end up in the hands of minors, we predict that overall youth consumption will remain stable. On net, we predict the public-health benefits of legalization to be positive.”

Notably, this commentary and the research it emphasizes appears only to consider the public health benefits that could result from folks substituting marijuana use for alcohol use. I have long thought that another possible public health benefit could flow from marijuana legalization if some heavy cigarette smokers end up smoking less in total because they sometimes substitute a few joints for a few packs of cigs. Similarly, one might further speculate that there might be a positive "reverse gateway" effect from marijuana legalization with respect to other dangerous drug use and abuse: perhaps fewer folks will try using, or end up harmfully abusing, harder drugs like ecstasy and heroin and meth and oxycodone if they can get always get a cheap and legal buzz from marijuana.

Of course, a lot of research about the use and abuse of various drugs will be needed in order to come to dependable conclusions about the full public health impact of modern marijuana reform developments. Still, especially when everyone is understandably all worked up about the Obamacare roll-out and broader health care reform realities, it is fun to speculate that modern marijuana reforms could end up being the most consequential and positive public health development of the Obama era.

I have long been drawn to the marijuana legal reform movement due to my general affinity for expanding personal freedom and my generally disaffinity for big-government programs like the war on drugs that seem very costly and mostly ineffective. But I have always respected the concerns expressed by serious people that pot prohibition is a public health necessity and that even modest moves toward marijuana legalization could prove costly and harmful in various ways. Without getting too much into the weeds of an empirical debate, I wonder if those who are vigorously opposed to (or even just generally resistant to) marijuana reform movements would still oppose reform if (and when?) empirical evidence starts to show that (some? many? all?) US public health measures and metrics are improved in the wake of marijuana legalization reforms.